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4
Unit 4: The Natural Environment and Species Survival
Photosynthesis 2
The light-dependent reactions make ATP and reduced NADP which are then used in
the light-independent reactions (Calvin cycle). The reduced NADP provides reducing
power (electrons or hydrogen) and the ATP provides the energy for the process of
making carbon dioxide into carbohydrate.
The key steps in the Calvin cycle are shown in the diagram.
1 Carbon dioxide combines with a 5-carbon
2 The 6-carbon compound formed is unstable and
compound called ribulose bisphosphate (RuBP).
immediately breaks down into two 3-carbon molecules,
This reaction is catalysed by the enzyme ribulose
glycerate 3-phosphate (GP).
bisphosphate carboxylase (RuBISCO), the
3 This 3-carbon compound is
most abundant enzyme in the world.
6C02
reduced to form a 3-carbon
sugar phosphate called
glyceraldehyde 3-phosphate
(GALP). The hydrogen for the
5 Ten out of every 12
reduction comes from the
GALPs are involved in
6RuBP (5C)
reduced NADP from the light12GP (3C)
the recreation of RuBP.
dependent reactions. ATP from
The ten GALP molecules
the light-dependent reactions
12ATP
12reduced
rearrange to form six
provides the energy required
NADP
5-carbon compounds;
for the reaction.
then phosphorylation
12NADP
12ADP + 12Pi
using ATP forms RuBP.
6ADP
12GALP
(3C)
(10GALP)
(2GALP)
6ATP
glucose (6C)
(hexose)
Outline of the Calvin cycle.
4 Two out of every 12 GALPs formed
are involved in the creation of a
6-carbon sugar (hexose) which can be
converted to other organic compounds,
for example amino acids or lipids.
ATP
Adenosine triphosphate (ATP) provides energy for chemical reactions in the cell.
When energy is needed, phosphate is removed from the ATP to give ADP and a
phosphate. The energy is released when the phosphate forms bonds with water. In the
photosynthesis light-dependent reactions, ATP is made using energy from light.
ATP
Be careful when discussing NADP,
what it is and what it does.
• NADP has electrons/hydrogen
added to it in the light-dependent
reactions of photosynthesis. It
becomes reduced NADP.
• NADP and ADP are not related in
photosynthesis and do different
things. One does not get converted
into the other.
ADP + Pi + energy
In photosynthesis, the ATP made is used as a source of energy in the light-independent
reactions. ATP is also used widely in organisms as a way of transferring energy. It is an
intermediate between energy-producing reactions and those that need energy.
Some of the glucose made in the Calvin cycle is used by the plant in respiration. The
rest is used to synthesise all the molecules on which the plant relies, for example other
simple sugars, polysaccharides, amino acids, lipids and nucleic acids.
nucleic
acids
(DNA and
RNA)
used in respiration to
produce carbon dioxide,
water and energy
plus phosphates and
nitrates from the soil
glucose from Calvin cycle
plus nitrate and
sulfur from the soil
starch (storage),
cellulose (wall)
Diagram showing some of the
fates of the glucose made in
photosynthesis.
10
lipids
(waterproofing
and storage)
amino acids
(to make
proteins)
proteins
(enzymes, and
in membranes)
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Topic 5: On the wild side
Where does photosynthesis happen?
In all eukaryotic cells there are membrane-bound structures called organelles. These
are the sites of specialised processes within the cell. For photosynthesis, plant cells
have a structure called the chloroplast. The diagram shows the functions which each
part carries out.
DNA loop – chloroplasts
Thylakoid membranes – a system of
contain genes for some
interconnected flattened fluid-filled
sacs. Proteins, including photosynthetic of their proteins.
pigments and electron carriers, are
embedded in the membranes and are
involved in the light-dependent reactions.
Stroma – the fluid surrounding
the thylakoid membranes.
Contains all the enzymes needed
to carry out the light-independent
reactions of photosynthesis.
Thylakoid space – fluid within
the thylakoid membrane sacs
contains enzymes for photolysis.
Granum – a stack of
thylakoids joined to
one another. Grana
(plural) resemble
stacks of coins.
Starch grain – stores
the product of
photosynthesis.
A smooth outer membrane – which
is freely permeable to molecules
such as CO2 and H2O.
Much of what is discussed in this
topic can be summarised in the
form of diagrams or flowcharts. But,
just reproducing such a diagram in
an examination will rarely lead to
full marks. Unless the diagram is
supported by some text it will not
demonstrate your understanding of the
topic. If you have a good memory and
have remembered, for example, the
Calvin cycle diagram accurately, use
this as a basis for your answer, not
your answer!
A smooth inner membrane – which contains many transporter
molecules. These are membrane proteins which regulate the
passage of substances in and out of the chloroplast. These
substances include sugars and proteins synthesised in the
cytoplasm of the cell but used within the chloroplast.
Q1
There are two steps where ATP is used in the Calvin cycle. Where are they?
Q2
Whereabouts in the Calvin cycle is RUBISCO used and what does it do?
Q3
Copy and complete the table of chloroplast functions below by suggesting,
for each structure within the chloroplast, the features that are adapted for
these functions.
Structure
Function(s)
thylakoid membrane
light-dependent reactions
thylakoid space
photolysis of water
granum
provides a site for light-dependent reactions
stroma
light-independent reactions
outer membrane
fully permeable
inner membrane
permeable to many substances which need to
enter or leave the chloroplast
Features
Thinking Task
Q1 Look at the diagram of the Calvin
cycle on page 10. Work out how
many carbon atoms are involved
at each stage (RuBP, CO2, GP,
GALP, glucose).
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Unit 4: The Natural Environment and Species Survival
Decay and decomposition
Decay and decomposition, no matter how distasteful it might sometimes seem to us,
is vital for the continuation of life on Earth. Plants need nutrients such as nitrogen,
potassium, phosphorus and carbon to make biomass. These nutrients are locked
into the tissues of the plants and any animals that might eat them. Once the plant
or animal dies the nutrients can be released only through decay. The process of
decomposition allows the nutrients to be recycled.
Micro-organisms are crucial to the decomposition process. The carbon cycle is a good
example of how nutrients are recycled and how micro-organisms help. Bacteria and
fungi produce a range of enzymes that are released on to the dead organic matter.
The products of external digestion are absorbed by the micro-organism and broken
down in microbial respiration, releasing carbon dioxide back into the atmosphere
where it can be used again in photosynthesis.
CO2 in air
and water
es
i
respiration
io
n
re
sp
ira
t
carbonate
rocks
carbon compounds
in decomposers
io
at
nt
e
im
sed
n
h
dea
t
exc h and
reti
on
pho
tos
yn
th
t
dea
dead organic
matter
vity
cti
carbon in
fossil fuels
carbon
compounds
in animals
feeding
v
a
nic
ca
ol
n
tio
on
irati
resp
carbon
compounds
in plants
s
bu
s
wea
the
rin
co
ga
m
nd
fossilisat
ion
The parts of the carbon cycle
which rely on micro-organisms
are shown by the orange arrows
on this diagram.
CSI Biology!
It is certainly possible to find out how long ago a mammal died. There are five main
ways that scientists go about this.
The time taken for the stages of
the blowfly life cycle at 21 °C.
The stage of larvae on a dead
body can be used to estimate
time of death.
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Topic 6: Infection, immunity and forensics
Indicator of time of death
How a forensic scientist uses the information
body temperature
Body temperature is usually 37 °C but the body begins to
cool straight after death. During the first 24 hours after death
the temperature of the body when it is found can be used to
work out how long ago the person died.
degree of muscle contraction
After death, muscles usually totally relax and then stiffen. This
stiffening is called rigor mortis. This happens within about
6–9 hours (depending on temperature). The stiffness occurs
because muscle contraction relies on ATP, which cannot be
made once respiration has stopped. So the muscles become
fixed. The stiffness wears off again after about 36 hours in
cooler conditions as the muscle tissue starts to break down.
extent of decomposition
Bodies usually follow a standard pattern of decay. Enzymes
in the gut start to break down the wall of the gut and then
the surrounding area. As cells die they release enzymes which
help to break down tissues. The signs of decomposition, such
as discoloration of the skin and gas formation, combined with
information about environmental conditions allow time of
death to be estimated.
forensic entomology
Determining the age of any insect maggots on the body
allows the time the eggs were laid to be determined. This
provides an estimate of time of death assuming any eggs were
laid soon after death.
stage of succession
As a body decays, the populations of insects found on it
change. There is a succession of species. The community of
species present when the body is found allows the stage of
succession to be determined and time of death estimated.
Putting all this information together can give the forensic scientist a very good estimate
of time of death.
4
All the methods used to indicate
time of death of a body are
subject to error. Remember this in
examinations where you are asked
to do calculations. Realistically, the
answer will often be in the form of
a range of possible times rather
than a precise figure.
Thinking Task
Q1
List three indicators that can be used to work out time of death.
Q2
What role do micro-organisms play in the carbon cycle?
Q1 Describe two ways in which
succession on a body is similar
to succession on a sand dune
or other natural system and one
way in which they are different.
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Unit 5: Exercise and Coordination
Muscles and movement
Muscles, joints and movement
Bones can move in relation to one another at joints. Different types of joint allow
different degrees of movement. Ligaments are made of elastic connective tissue.
They hold bones together and restrict the amount of movement possible at a joint.
Tendons are cords of non-elastic fibrous tissue that anchor muscles to bones.
bone
TENDON
s JOINSMUSCLE
TOBONE
cartilage
s ABSORBSSYNOVIAL
FLUID
s ACTSASSHOCK
ABSORBER
MUSCLE
LIGAMENT
s JOINSBONETOBONE
s STRONGANDFLEXIBLE
PADOFCARTILAGE
s GIVESADDITIONAL
PROTECTION
SYNOVIALMEMBRANE
s SECRETESSYNOVIALFLUID
FIBROUSCAPSULE
s ENCLOSESJOINTS
SYNOVIALFLUID
s ACTSASLUBRICANT
A typical synovial joint.
Skeletal muscles are those attached to bones and are normally arranged in
antagonistic pairs. This means that there are pairs of muscles which pull in opposite
directions. Flexors contract to flex, or bend a joint, e.g. biceps in the arm; extensors
contract to extend, or straighten a joint, e.g. triceps in the arm.
Remember that muscles can’t
stretch themselves. It is the pull
created by the contraction of the
antagonistic muscle that stretches a
muscle when it is in a relaxed state.
The prefix myo- refers to ‘muscle’
and sarco- to ‘flesh’ (i.e. muscle) so
specialist terms starting with myoor sarco- will refer to structures
within muscles.
Each skeletal muscle is a bundle of millions of muscle cells called fibres. Each muscle
cell may be several centimetres long and contains several nuclei. It contains many
myofibrils which are made up of the fibrous proteins actin (thin filaments) and
myosin (thick filaments). The cell surface membrane of a muscle cell is known as the
sarcolemma. The sarcoplasmic reticulum is a specialised endoplasmic reticulum
which can store and release calcium ions. The cytoplasm inside a muscle cell is called
the sarcoplasm. The specialised synapse (see page 63, Topic 8) between neurones
and muscle cells is called the neuromuscular junction.
The sliding filament theory of muscle
contraction
The functional unit of a muscle
fibre is called a sarcomere.
When the muscle contracts
the thin actin filaments move
between the thick myosin
filaments, shortening the length
of the sarcomere and therefore
shortening the length of the
muscle.
A
one sarcomere
myosin
actin
B
The arrangement of actin and myosin
filaments in a sarcomere when relaxed
(A) and contracted (B).
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Topic 7: Run for your life
5
Myosin filaments have flexible ‘heads’ that can change their orientation, bind to actin
and hydrolyse ATP (using ATPase). Actin filaments are associated with two other
proteins, troponin and tropomyosin, that control the binding of the myosin heads to
the actin filaments.
When a nerve impulse arrives at a neuromuscular junction, calcium ions are released
from the sarcoplasmic reticulum and the following events take place that lead to the
contraction of the muscle.
Ca2+ binding site
tropomyosin
troponin
ADP
Pi
Ca2+ attaches to actin
troponin (on the
actin) causing it
to move together
with the threads
of tropomyosin.
Myosin
binding sites
blocked by
tropomyosin.
Myosin head
cannot bind
myosin
binding site
Ca2+
Ca2+
AtDP
Pi
Myosin binding
sites on the actin
are exposed so
myosin forms
cross-bridges with
the actin filament.
The myosin heads
release the ADP
and Pi and change
shape as a result =
the power stroke.
Ca2+
Ca2+
Ca2+
AtDP
Pi
ATPase causes
ATP hydrolysis
ADP and Pi released
Ca2+
Ca2+
Ca2+
Myosin head
returns to
upright
position.
ATP binds
Ca2+
Ca2+
Ca2+
ATP
ADP + Pi
ATP binds
to the myosin
head causing
it to detach
from the actin.
The sliding filament theory of muscle contraction.
Characteristics of fast-twitch and
slow-twitch muscle fibres
Slow-twitch
Fast-twitch
specialised for slower, sustained contraction
and can cope with long periods of exercise
specialised to produce rapid, intense
contractions in short bursts
many mitochondria – ATP comes from aerobic few mitochondria – ATP comes from
respiration (electron transport chain)
anaerobic respiration (glycolysis)
lots of myoglobin (dark red pigment) to store
O2 and lots of capillaries to supply O2. This
gives the muscle a dark colour
little myoglobin and few capillaries. The
muscle has a light colour
fatigue resistant
fatigue quickly
low glycogen content
high glycogen content
low levels of creatine phosphate
high levels of creatine phosphate
Q1
Give one reason why fast-twitch muscles are more likely to get tired faster
than slow-twitch muscles.
Q2
Describe the role of ATP in muscle contraction.
Q3
Explain why muscles are arranged in antagonistic pairs.
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Topic 8: Grey matter
5
Brain development
We are born with a range of innate behaviours (behavioural responses that do not
need to be learnt) such as crying, grasping and sucking. However, the brain still needs
much growth and development after birth through the formation of synapses and the
growth of axons.
Evidence for critical windows
Critical windows (or critical periods) for development are those periods of time where it
is thought that the nervous system needs specific stimuli in order to develop properly.
Evidence for critical windows for development has come from medical observations
(e.g. children who develop cataracts before the age of 10 days may suffer from
permanent visual impairment even if the cataracts are repaired at a later date) and from
animal models. Hubel and Wiesel used kittens and monkeys as models to investigate
the critical window in visual development because of the similarity of their visual systems
to that of humans.
The animals were deprived of the stimulus of light into one eye (monocular deprivation)
at different stages of development and for different lengths of time. They found that
kittens deprived of light in one eye at 4 weeks after birth were effectively permanently
blind in that eye. Monocular deprivation before 3 weeks and after 3 months had
no effect. It was thought that during the critical period (about 4 weeks after birth)
connections to cells in the visual cortex from the light-deprived eye had been lost. This
meant that the eye that remained open during development became the only route for
visual stimuli to reach the visual cortex.
Eye deprived of light during critical
window
Eye that remains open during the critical
window
Axons do not pass nerve impulses to cells in
the visual cortex.
Axons pass nerve impulses to cells in the
visual cortex.
Inactive synapses are eliminated.
Synapses used by active axons are
strengthened.
Eye has no working connection to the visual
cortex and is effectively blind, even though
the cells of the retina and optic nerve work
normally when exposed to light.
Synapses only present for axons coming from
the light-stimulated eye. So the visual cortex
can only respond to this eye.
Issues about the use of animals for research
The use of animals as models for understanding how humans develop, or how
new drugs may affect us, is a very controversial area. There are those who hold an
absolutist view of animal rights and think we should never keep animals or use
them in medical research. From the point of view of medical research, a much more
widespread position is the relativist view that humans should treat animals well and
minimise harm and suffering so far as is possible. Here the emphasis is on animal
welfare, respecting their rights to such things as food, water, veterinary treatment
and the ability to express normal behaviours. This is pretty much the position in
European law. This all assumes that animals can suffer and experience pleasure.
A utilitarian ethical framework allows certain animals to be used in medical
experiments provided the overall expected benefits are greater than the overall
expected harms based on the belief that the right course of action is the one that
maximises the amount of overall happiness or pleasure in the world.
Visual development is an
example of how the effects of
nature and nurture can combine
in development. The genes
control the development of the
responsive cells in the visual
cortex (nature) but a stimulus from
the environment is needed during
the critical window for the correct
connections to be made (nurture).
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Unit 5: Exercise and Coordination
The role of nature and nurture in brain
development
• Nature: Many of our characteristics develop solely under the influence of our
genes with little influence from our environment or learning, e.g. blood group.
Although it is not generally
possible to experiment on people,
it is possible to select a sample
carefully so as to ensure that nonexperimental variables, such as
age and sex, are matched so it is
more like a traditional controlled
experiment in the laboratory.
• Nurture: Many characteristics are learnt or are heavily influenced by the
environment, e.g. how long your hair is.
Most of our characteristics are actually determined by nature and nurture or nature via
nurture. We are the result of a mixture of genetic and environmental factors. Human
behaviours, attitudes and skills may have an underlying genetic basis but are modified
by experience or the environment in a way which is very complex. For example, the
chance of developing some diseases, such as some cancers, has a genetic basis,
where a gene or several genes interact to confer susceptibility to the disease with
environmental factors contributing to the risk of developing the disease.
Evidence for the relative roles of nature and nurture in brain development come from
a variety of sources:
• The abilities of newborn babies: Newborn babies have some innate capacities.
These suggest that genes help to form the brain and some behaviours before the
baby is born.
• Studies of individuals with damaged brain areas: Some patients who have
suffered from brain damage show the ability to recover some of their brain
function. This demonstrates that some neurones have the ability to change.
• Animal experiments: e.g. Hubel and Weisel’s experiments on critical windows for
sight, suggest that external stimulation is important in brain development.
• Twin studies: Identical twins share all the same genes. Fraternal (non-identical)
twins share the same number as any other sibling would. Twin studies can help to
estimate the relative contribution of genes and the environment. Any differences
between identical twins must be due to the effects of the environment.
Identical twins raised apart in comparison to those raised together are particularly
useful for study. For example if there is a greater difference between those twins
raised apart than twins raised together it suggests some environmental influence.
However, twins raised apart may not have completely different environments and
twins raised together may develop different personalities due to a desire to be
different. In general if genes have a strong influence on the development of a
characteristic, then the closer the genetic relationship, the stronger the correlation
will be between individuals for that trait.
• Cross-cultural studies: Investigations into the visual perception of groups
from different cultural backgrounds support the idea that visual cues for depth
perception are at least partially learnt.
Thinking Task
Q1 What is your personal view on
the use of animals in medical
research? For example, how
many a fruit flies, b mice, c cats,
d monkeys do you think you
could use to test new drugs to
help treat i breast cancer, ii
malaria, iii wrinkles in the skin?
How do you justify your position?
68
Q1
Describe why it may be dangerous to leave a patch over the damaged eye
of a child for a prolonged period of time.
Q2
Explain why kittens and monkeys have been used in experiments looking
at human brain development.
Q3
If one identical twin has schizophrenia there is 80% chance that their twin
will also have symptoms of schizophrenia. However, if one fraternal twin
has schizophrenia there is only a 15% chance that their twin will also have
symptoms of schizophrenia. What do these figures suggest about the
contribution of nature and nurture on the development of schizophrenia?
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